TABLE OF CONTENTS The term client server was introduced in the 1980s to describe the relationship between computers on a network. The term gained broad adoption in the 1990s and is widely used to refer to processing performed between a series of remote computers with a larger, more powerful central computer, usually through a message-based communication protocol, such as TCP/IP. The server provides services to one or more clients, such as access to database data.
A special case of client server processing occurs when the client runs on the same machine as the server. This is known as local client processing.
In general, the client server model is known as a two-tier architecture, where the client resides on the first tier, and the server is the second heavier tier. Two-tier models tend to scale well up to roughly 100 clients. Beyond that contention issues typically require expansion of the server to a shared-nothing architecture of some sort, or the introduction of a third tier.
To resolve the limitations of two-tier architectures the industry began to adopt a number of three-tier architectures, as shown in Figure 13.4. These architectures separate the workload balancing and transaction flow from the database layer. The client tier and the database tier remain, but between them a new tier is inserted. The middle tier, often called the application server, includes the bulk of the application processing logic as well as transaction routing and queuing technology. The transaction handling usually also supports an asynchronous mode.
